Not applicable.
Not Applicable.
The preferred embodiments of the present invention generally relate to medical diagnostic imaging systems, and in particular relates to a method and apparatus for correcting electronic offset and gain variations in medical imaging systems employing solid state detectors.
X-ray imaging has long been an accepted medical diagnostic tool. Xray imaging systems are commonly used to capture, as examples, thoracic, cervical, spinal, cranial, and abdominal images that often include information necessary for a doctor to make an accurate diagnosis. X-ray imaging systems typically include an xray source and an x-ray sensor. When having a thoracic x-ray image taken, for example, a patient stands with his or her chest against the x-ray sensor as an x-ray technologist positions the x-ray sensor and the x-ray source at an appropriate height. X-rays produced by the source travel through the patient""s chest, and the x-ray sensor then detects the x-ray energy generated by the source and attenuated to various degrees by different parts of the body. An associated control system obtains the detected x-ray energy from the x-ray sensor and prepares a corresponding diagnostic image on a display.
The x-ray sensor may be a conventional screen/film configuration, in which the screen converts the x-rays to light that exposes the film. The x-ray sensor may also be a solid state digital image detector. Digital detectors afford a significantly greater dynamic range than conventional screen/film configurations.
One embodiment of a solid state digital x-ray detector may be comprised of a panel of semiconductor FETs and photodiodes. The FETs and photodiodes in the panel are typically arranged in rows (scan lines) and columns (data lines). A FET controller controls the order in which the FETs are turned on and off. The FETs are typically turned on, or activated, in rows. When the FETs are turned on, charge to establish the FET channel is drawn into the FET from both the source and the drain of the transistor. Due to the imperfect nature of the amorphous silicon FETs, the charge is retained temporarily when the FET is turned off and bleeds out, decaying, over time. which corrupts desired the signal in the form of an offset. The source of each FET is connected to a photodiode. The drain of each FET is connected to readout electronics via data lines. Each photodiode integrates the light signal and discharges energy in proportion to the x-rays absorbed by the detector. The gates of the FETs are connected to the FET controller. The FET controller allows signals discharged from the panel of photodiodes to be read in an orderly fashion. The readout electronics convert the signals discharged from photodiodes. The energy discharged by the photodiodes in the detector and converted by the readout electronics is used by an acquisition system to activate pixels in the displayed digital diagnostic image. The panel of FETs and photodiodes is typically scanned by row. The corresponding pixels in the digital diagnostic image are typically activated in rows.
The FETs in the x-ray detector act as switches to control the charging of the photodiodes. When a FET is open, an associated photodiode is isolated from the readout electronics. The associated photodiode is discharged during an x-ray exposure. When the FET is closed, the photodiode is recharged to an initial charge by the readout electronics. Light is emitted by a scintillator in response to x-rays absorbed from the source. The photodiodes sense the emitted light and are partially discharged. Thus, while the FETs are open, the photodiodes retain a charge representative of the x-ray dose. When a FET is closed, the voltage across the photodiode is restored to re-establish a desired voltage across the photodiode. The measured charge amount to re-establish the desired voltage becomes a measure of the x-ray dose integrated by the photodiode during the length of the x-ray exposure.
Readout electronics read the output signal from the x-ray detector panel. When the readout electronics are activated to read out the output signal from the x-ray detector panel, an electronic offset may be added to the resulting image. For example, some excess charge may xe2x80x9cleakxe2x80x9d from the readout electronics and add to the output signal. The charge leakage from the readout electronics may induce structured artifacts (including ghost images or distortions) in the x-ray image. The offset, such as charge leakage, from the readout electronics can be measured initially by acquiring a xe2x80x9cdarkxe2x80x9d image. A xe2x80x9cdarkxe2x80x9d image is a reading done without x-ray exposure. A xe2x80x9cdarkxe2x80x9d image simply activates the FETs on the x-ray detector panel and reads the output signal through the readout electronics. Thus, a xe2x80x9cdarkxe2x80x9d image may determine the offset, such as charge leakage, from the FET controller readout electronics. By subtracting the xe2x80x9cdarkxe2x80x9d image pixel value from the actual xe2x80x9cexposexe2x80x9d x-ray image pixel value of a desired object, the offset (i.e., charge leakage) effects from sources such as the readout electronics may theoretically be eliminated.
Gain calibration is performed on the detector and electronics in order to provide gain correction coefficients for the x-ray image on a pixel by pixel basis. Gain calibration includes the sensitivity of the detector and the gain of the readout electronics. A flat field uniform x-ray exposure, with only an x-ray calibration phantom that uniformly attenuates the exposure, is used for gain calibration. Thus, it is desirable to perform gain calibration infrequently. After exposure, pixels in the gain calibration image are examined. Pixels that have a small response (less than the mean) are multiplied by a factor greater than one. Pixels that have a large response (greater than mean) are multiplied by a factor less than one. Pixels that exhibit a response below a given threshold are mapped out as xe2x80x9cdeadxe2x80x9d pixels. Pixels above a second given threshold are also mapped out. Pixels above the second threshold will probably saturate too easily. Pixels that saturate too easily will probably not return any additional signal, exhibiting limited dynamic range.
X-ray images may be used for many purposes. For instance, internal defects in a target object may be detected. Additionally, changes in internal structure or alignment may be determined. Furthermore, the image may show the presence or absence of objects in the target. The information gained from x-ray imaging has applications in many fields, including medicine and manufacturing.
In any imaging system, x-ray or otherwise, image quality is of primary importance. In this regard, x-ray imaging systems that use digital or solid state image detectors (xe2x80x9cdigital x-ray systemsxe2x80x9d) face certain unique difficulties. Difficulties in a digital x-ray image could include image artifacts, xe2x80x9cghost images,xe2x80x9d or distortions in the digital x-ray image. One source of difficulty faced by digital x-ray systems is offset (i.e., electronic leakage) and gain variation of readout electronics used in digital x-ray systems.
In an ideal image adjustment, offset correction may be performed as described above by subtracting the value of a xe2x80x9cdarkxe2x80x9d image pixel from the value of a corresponding pixel in an exposed x-ray image. The result may be multiplied by a gain calibration coefficient described above. However, variation in gain and offset in readout electronics may affect offset correction and gain calibration.
Changes in temperature may have an effect on readout electronics. The output signals from the x-ray detector panel are very small. Since the output signals are very small, readout electronics are very sensitive. The sensitive readout electronics are susceptible to changes in temperature. Differences in temperature at different times will affect the signal read out by the readout electronics at the different times. Differences in temperature between gain calibration and at the time exposure data is read may cause variations in the gain to corrupt measurements taken when the image data is read from the x-ray detector panel. If the gain of the readout electronics changes between gain calibration and the x-ray image, the gain correction will be in error. Similarly, differences in temperature may cause changes in the offset, such as the amount of charge that xe2x80x9cleaks,xe2x80x9d from the readout electronics when it is activated to read the output signal from the x-ray detector panel. As a result, the offset from the readout electronics in the x-ray image may differ from the offset from the readout electronics in the xe2x80x9cdarkxe2x80x9d image. If the offsets from the readout electronics differ, the structured artifacts induced by the readout electronics offset (i.e., electronic leakage) will not be eliminated by subtracting the xe2x80x9cdarkxe2x80x9d image from the actual xe2x80x9cexposedxe2x80x9d xray image of a desired object.
As noted above, the characteristics of digital image detectors inherently vary. Although there is a need to provide consistent and accurate image quality (and in particular, image gray scale resolution) within and across multiple medical diagnostic imaging systems, in the past there has been no automated technique for providing such consistency.
Thus, a need exists for a method and apparatus for correcting electronic offset and gain variations in a solid state x-ray detector.
A preferred embodiment of the present invention provides a method and apparatus for correcting electronic offset and gain variations in solid state x-ray detectors. The method and apparatus include adding two or more rows to the end of a normal x-ray detector scan area. The additional rows may be outside the physical image area of a solid state x-ray detector. The additional rows then may be used to measure the xe2x80x9csignalxe2x80x9d induced by variations in electronic offset (such as electronic leakage) that may occur between xe2x80x9cdarkxe2x80x9d image acquisition and x-ray image acquisition in a solid state x-ray detector. The additional rows also may be used to measure the variations in gain that may occur between gain calibration and x-ray image acquisition. The measurements may be made at the end of a detector scan. The signals induced by variations in electronic offset and gain might otherwise cause visible structured artifacts in the x-ray image.
An alternative preferred embodiment may use an existing solid state x-ray detector scan area and simply not activate two or more rows at the end of the x-ray detector scan area. This embodiment may reduce the image area covered by the scan.